The present invention relates generally to graphic or image display systems, and, more particularly, to a method for enhancing the visual realism of the terrain or topography of a scene being displayed, and an image display system which implements this method.
Graphic or image display systems are currently used in flight simulators to enable pilots and other aircraft crew or trainees (hereinafter collectively referred to as "mission personnel") to view a visual image of the terrain or topography of a geographical area being flown over. The imagery can be generated on a real-time basis using image data generated by FLIR (Forward Looking InfraRed) and/or RADAR systems and/or using image data derived from digital map systems (e.g., moving maps) and/or digitized photographs and/or other digital image databases of landmass scenes.
In general, it is desirable to have the capability to display perspective and plan views of terrain which are as highly-detailed and realistic as possible in order to increase the situational awareness of mission personnel, and thereby their effectiveness. However, the visual images of landmass terrain in many presently available image display systems are generated in such a manner that the topographical features of the terrain are constructed of polygon faces having a constant normal vector across their entire extent. The polygon faces constituting the terrain features are stored in a polygonal terrain database which is correlated in scene content and elevation with all other visual, FLIR, moving map, and RADAR databases utilized by the system to generate the displayed images.
The result is that the elevational features of the terrain are "cartoonish-looking" on conventional visual display devices, as can be seen in FIG. 1, which depicts a scene displayed by a conventional RADAR display system. Typically, the polygon faces appear as large triangles of constant intensity and sides having a scale length of 1/2 nautical mile.
Various image display systems have been proposed for generating higher resolution, more realistic visual displays of landmass scenes. Several of these proposed systems are discussed below.
U.S. Pat. No. 4,766,555, issued to Bennett, discloses a computer-generated visual display for a flight simulator which includes a data processing circuit which generates high detail realistic images statistically, under the control of a control database. More particularly, the landscape "character" data is stored in the control database, and based on a statistical analysis of that data, various "cultural features" (e.g., secondary roads, ponds, farms, trees, and so on) stored in another database are combined with the landscape "character" data, to thereby produce a final database which is utilized in the generation of the visual images. Although the Bennett system may enhance the realism of image displays used in flight simulators, it does not address the problem of "cartoonish-looking" images generated by an image display system which utilizes a polygonal terrain database.
U.S. Pat. No. 4,985,854, issued to Wittenburg, and U.S. Pat. No. 5,353,030, issued to Koch et al., disclose image display systems which overlay photographic imagery on a terrain database in order to enhance the realism of the image display. In this regard, Wittenburg rapidly generates photo-realistic imagery from a digitized down-looking photograph which has been combined and registered with digital terrain elevation data.
U.S. Pat. No. 5,317,689, issued to Nack et al., discloses a computer image generation (CIG) system which uses a three-dimensional voxel database formed by combining three-dimensional digital terrain elevation data with two-dimensional plan view and oblique view aerial photography digital data, to thereby generate more realistic images.
Although these image display systems may also enhance the realism of image displays used in flight simulators, they require a database generated from digitized photographs, whereas many RADAR display and other systems rely upon polygonal terrain databases of the type described hereinabove in order to provide the required correlation between the visual, RADAR, sensor imagery, moving map, and other databases used in generating the RADAR display. Thus, the Wittenburg and Koch et al. systems do not provide a solution to the problem of "cartoonish-looking" images generated by an image display system which utilizes a polygonal terrain database.
U.S. Pat. No. 5,367,615, issued to Economy et al., discloses a computer image generation system in which the number of vertices which define each of a plurality of terrain-defining polygons is augmented in real time for providing finer detail and for effecting substantially continuous smooth level of detail (LOD) transitions. Some vertex components may have predetermined values derived from mapping data or from other deterministic sources and may be stored in compact form. The level of detail of features such as terrain can be increased by increasing the density of polygons which represent the terrain features. However, each of the polygon faces has a constant normal vector across their entire extent, and thus, the problem of "cartoonish-looking" images generated by an image display system which utilizes a polygonal terrain database is not solved. Moreover, the level of resolution or detail which is afforded by this technique is limited to that of the original polygonal terrain database, as no data from a higher detail terrain database is used.
Based on the above, it can be appreciated that there presently exists a need in the art for a method for enhancing the resolution and visual realism of an image generated using a polygonal terrain database, and an image display system which implements this method. The present invention fulfills this need in the art.